Diffusion vacuum pump



April -27, 1937.

FL E. BANCROFT DIFFUSION VACUUM PUMP Filed Nov. 5, 1935 all Ill;

Ihvehtcjr Frank E. Bancroft by V v is AbtOP'neg Patented I Apr. 27, 1937aovarss DIFFUSION VACUUM PUMP Frank Edmund Bancroft, Withington,Manchester, England, assignor to Metropolitan- Vickcrs ElectricalCompany, Limited, a company of Great Britain Application Novembe 5,

1935, Serial No. 48,405

In Great. Britain November 9, 1934 3 Glaims. (Cl. 230-101) Thisinvention relates to improvements in diffusion vacuum pumps of thecondensation or difiusion type. It is particularly applicable to pumpsof the type described in British, Patent No. 346,293 which are intendedfor use with a low vapor pressure oil as a working fluid.

One object of the-present invention is to .pro-

vide a high speed high vacuum single jet diffusion pump which isefiicient, reliable and of simple and relatively cheap construction andcapable of giving pressures less than 10- mm.

I for use in the continuous, evacuation oflarge I demountable apparatussuch as thermionic valves and X-ray tubes. Other contemplatedapplications include use with induction furnaces,

stills and the like.

To enable the invention to be clearly understood one convenientarrangement thereof by way of example is illustrated in the accompanyingdrawing and described with reference thereto, the single figure being asectional side elevation of the vacuum pump.

While my improved pump is by no means limited to use with a particularworking fluid, it has been found to possess unusually favorablecharacteristics for use with the low vapor pressure oil commerciallyavailable under the registered trade-mark name, .Apiezon. This comprises a highly refined oil possessing the quality of being able to beheated in vacuum sufficiently to raise its vapor pressure to about 1 or2 mm. without cracking. Since its vapor pressure becomes extremely lowat temperatures not much below room temperature, it is possible toemploy it to .obtain a very positive jet action without resorting toexcessive cooling for condensation purposes.

Referring to the drawing, l is the outer shell of the. pump, this shellcomprising a tubular, porton of cylindrical section and, at its lowerend, a conical portion la, the cylindrical portion I having the lateralforevacuum connection pipe 2 near its upper end.'- The smaller diameterof the conical portion la of the shell is approximately half thediameter of the cylindrical portion I of the shell. Secured to the lowerend of the conical portion la is the boiler 3 beneath which is disposedthe heater which may be a hot plate applied to the flat underside of theboiler. 50 In the arrangement illustrated said heater comprises aplurality, conveniently three, of heater elements, of which one isillustrated at 4, this element being of a known form and comprising ahollow core of insulating material carrying an 55 external wire winding.Each such heating element 4 is adapted to be located in a tube 5 weldedbeneath the boiler 3 and to be retained in the tube 5 by means of abayonet fastening indicated by dotted lines at 6a, the slot of thefastening being provided in a detachable cap I, having a bushedperforation 8 through which the electrical leads for the heater 4 pass.The three (or more) tubes 5 may be enclosed in a detachable shell 6which may be chromiumplated for the purpose of retaining the heat.

Resting upon' the floor of the boiler 3, and engaged with the latter bymeans of friction studs 9, .is the vapor uptake tube l0 having adiameter only slightly smaller than that of the lower end of the conicalportion Ia of the outer shell. The vapor uptake tube I0 is surmounted bythe cowl H, and above the latter the molecule baffle l2, these membersbeing secured to a boss 83 which in turnis secured to the centre rod Il,-

-which at its lower end is secured to the centre of the boiler 3. Thevapor uptake tube I 0 and the supporting rod It are centred with respectto each other by means of a tube [5 carried by a spider, two of theradial spokes of which are indicated at It.

The condenser tube l'l' depends into and divides the annular spacebetween the main outer shell I and the vapor uptake tube l0. Saidcondenser tube is attached to or more con- 'veniently, as in thearrangement illustrated, is

integral with the pump intake 11a, The tube Ila is provided with anexternal flange I8 which is adapted to be demountably vacuum-sealed tothe flange i9 provided at the top of the main outer shell I. The vacuumseal is preferably completed'by means of a gasket 20 of appropriatematerial and an externalv flood of oil in a. channel 2| provided abovethe flange I9 by the cylindrical flange 22 which may be provided with adrain tap or with the illustrated drain plug 23.

The condenser tube H has a water. jacket, which in the illustratedarrangement comprises a tube 24 arranged in double or re-entrant helicalform around the outer surface of the tube I1, and connected thereto suchas by welding or soldering in a manner to provide good thermal contact.It will be obvious that the water jacket might be alternatively providedby means of an additional shell secured coaxially to the tube l1.

The ends of the tube 24, which is preferably of flattened section asshown, are carried through perforations in the flange l8, as indicatedat.24a. Within the condenser tube I1 is flxed the molecule baflie 25 inthe form of an internalfiange, this bafiie in conjunction with thebaflle I2 serving to prevent molecules of working fluid vapor passinginto the evacuating system, as described for example in British PatentNo. 346,293.

The lower edge of the condenser tube I1 is bevelled, as indicated at Hb,to facilitate the dripping of the condensed working fluid therefromclear of the conical baiiie Illa which is secured towards the lower endof the vapor uptake tube In within the conical portion In of the outershell, said bailie Ilia having a diameter slightly less than that of thecondenser tube H. The tube 10 is formed at its lower end with holes [bto permit the condensed working fluid to flow into the boiler.

It will be seen that the pump inlet of the condenser and its securingmeans comprises a unit which can readily be connected with the apparatusto be evacuated either direct or by means of a pipe 26 which may besecured in a dismountably vacuum-tight manner, conveniently by anarrangement which may be generally similar to the seal already describedcomprising the items l8--23.

The floor of the boiler 3 has a ribbed upper surface, the ribs beingarranged as two concentric circles, as' indicated at 3a and radialchannels (which are not visible in the drawing) are provided, namely, inorder that a uniform fluid level may be maintained within the boiler incase the fluid level should become insuflicient to flood the ribs 3a.Conveniently there are three such radial communicating channels arrangedat an angle of 120 with respect to each other.

The pump above described'and shown in the- .drawing is conveniently madeof mild steel with the exception of the vapor uptake tube l0, its cowlll, baille l2, and centre supporting rod H, which parts are preferablyof copper or dther metal having a relatively high thermal conductivity,for the purpose set forth in the aforesaid British Patent No. 346,293.

The pump as above described and shown in the drawing has actually beenconstructed and successfully operated, and the vertical height of thecondenser tube l1, excluding the thickness of the illustrated flange atits upper end, was 11 inches, the accompanying drawing being otherwisesubstantially to scale. The normal working level of the oil in theboiler 3 is indicated at 21, and the capacity at this level isapproximately 600 cc.

It will be understood that various minor modifications may be made inthe arrangement illustrated, all within the scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is

1. A single jet diifusion pump comprising a substantially cylindricalouter casing having an outlet for pumped gases adjacent the top thereof,said casing being inwardly tapered at the bottom to join with a circularheating member of substantially less diameter than said casing, a vaporuptake tube comprising a metal cylinder of substantially the samediameter as said member and projecting upwardly therefrom, a workingfluid injection orifice adjacent the top of said tube, a cylindricalcondenser unit projecting downwardly into the annular space between saidouter casing and said uptake tube, an inlet at the top of said pumpassembly for admitting pumped gases downwardly within said condenserunit, an external flange projecting laterally from said condenser unitfor supporting the same, and demountable means hermetically sealing saidflange to the upper portion of said casing.

2. A single jet diffusion pump comprising an outer casing having anoutlet for pumped gases adjacent the top thereof, said casing beinginwardly tapered at the bottom to join with a heating member ofapproximately one-half the diameter of the casing, a vapor uptake tubeof approximately the same diameter as said member and joined thereto, aworking fluid injection oriflce adjacent the top of said tube, acondenser including a metal shell projecting downwardly into the spacebetween said outer casing and said uptake tube, said condenser shellbeing outwardly beveled at its lower edge, a conical baffle of lessdiameter than the condenser shell secured to the vapor-uptake beneaththe lower edge of the condenser shell and an inlet at the top ofsaidpump for admitting pumped gases downwardly within said condenser shell.

3. A diffusion pump comprising an outer casing having an outlet forpumped gases adjacent the top thereof and being inwardly tapered at thebottom to join with a heating member of substantially less diameter thanthe casing, said heating member being provided with ribbedheatexchang'ing surfaces, a vapor uptake tube of substantially the samediameter as said member and projecting upwardly therefrom, a workingfluid injection orifice adjacent the top of said tube, a condenser unitprojecting downwardly into the space between said outer casing and saiduptake tube, an inlet at the top of said pump for admitting pumped gasesdownwardly within said condenser unit, an external flange projectinglatfor supplying cooling fluid to said condenser unit.

FRANK EDMUND BANCROFI.

